This invention has for its principal object the provision of a simplified, convenient and safe means to positively position a crane boom extension precisely at several different angularly offset use positions relative to the center line of the telescopic crane boom.
Another objective is to be able to accomplish the above offsetting adjustments of the boom extension at ground level in a semi-automatic mode by use of a very simple mechanism which can remain permanently attached to the boom extension.
Prior U.S. Pat. Nos. 3,831,771 and 3,968,884 show arrangements which permit the angular offsetting of boom extensions or jibs relative to the center line of a crane boom. In the former patent, the offsetting adjustments are achieved through the use of interchangeable variable length connecting links between the base of the boom extension and the boom nose on opposite sides of the pivot for the boom extension. In the latter patent, the offsetting adjustments of the boom extension are obtained by the use of guy lines and coacting struts which limit the extent to which the boom extension can be offset or tilted. It is necessary to connect the guy lines at different positions on the boom and boom extension to achieve the desired angular offsetting.
Both of these prior art arrangements tend to be awkward or inconvenient and neither possess the simplicity of construction and the semi-automatic mode of operation of the present invention at a near ground level position of the boom.
The above and other advantages of the invention over the prior art will become apparent during the course of the following description.